1. Field of the Invention
This invention relates to vertical axis wind turbine and in particular, refers to a device and method for adjusting angle-of-attack of blades which can effectively lower the starting wind speed and increase the wind energy utilization ratio.
2. Description of the Related Art
In order to make better use of wind energy, various wind generating devices of different structure and form have been developed for a long time. Depending on the position of the rotating axis, wind turbines are divided into horizontal axis wind turbines (HAWT) and vertical axis wind turbines (VAWT). Vertical axis wind turbines can be divided into lift-type and drag-type. The characteristics of lift-type and drag-type rotors are explained, for example, in “Wind Energy and its Utilization” (Energy Publishing House, February 1984, pp. 81-85.” Though both lift-type and drag-type rotors are vertical axis rotors, the principles behind rotation of the driving rotor are completely different and the effects are also different.
“Lift type” means that when wind blows over the surface of a blade, the wind speeds for blade's outer and inner surfaces are different due to blade's shape and angle of attack. A difference in wind speed is generated for blade's outer and inner surfaces. According to fluid mechanics, when the fluid speeds for inner and outer surfaces are different, a pressure difference is generated between two surfaces, i.e., lift force. When the blades are installed with a specific angle of installation (angle of attack) at different positions, this pressure difference (lift force overcomes drag force) will produce a driving moment around rotor's centre of gyration, which drives the rotor to rotate.
But when the rotor rotates, because the blade's azimuth angle constantly changes, the blade's angle of attack changes accordingly; at the same time, the relative speed between the blade and the wind also constantly changes, resulting in the change of the relative rotating angle between the blade and the wind. Therefore, the size and direction of the driving moment produced by the blades change at all times.
For a lift-type vertical rotor, the direction and size of the driving moment of the blades in the dissymmetrical rear half of the circumference are opposite to those in the front half of the circumference, and there is a large difference between their absolute values. “Positive work” is done in the front half of the circumference, while “negative work” is done in the rear half of the circumference. But because the wind flowing through the front half circumference is “cut” by the blades disposed on front half circumference, the wind speed is lowered. Because wind's energy is proportionate to the cube of wind speed, the absolute value of the driving moment received by the blades in rotor's rear half of the circumference is far below the driving moment received by the blades in front half circumference, thus making the rotor rotate. The higher the rotor's rotational speed, the bigger the wind speed difference and the bigger the power difference between the front and the rear half circumferences. Thus, the efficiency of lift-type rotor increases with the increase of rotor's rotational speed.
The drag-type vertical axis rotor has an essential difference from the lift-type rotor. The blade shape for drag-type rotor is different from that for lift-type rotor. Simply speaking, the blade of drag-type rotor can be a door plate. The drag coefficients on both sides of the blade are different; the side with a bigger drag coefficient obtains a bigger wind pressure. Wind pressure difference still makes the rotor rotate, but because drag-type rotor uses the component of wind's force vertical to wing surface, “positive work” is performed in the rotor's right half circumference, while “negative work” is done in rotor's left half circumference. Because the wind speeds at left and right sides are the same and the difference only lies in the drag coefficients on both sides of the blade, the work done in left and right half circumferences is the function of blade's drag coefficient times the cube of the blade's relative wind speed. If the wind speed is V and the linear speed of the rotor rotation is u: in right half circumference, because wind “drives” blade to move, the relative linear speed of the blade is lowered (V−u); and in left half circumference, because the blade moves upwind, the relative linear speed (V+u) is higher than that for the right half circumference. Therefore, when wind blows from the left, the left gate flap has the biggest moment and the blades obtain the biggest moment. The moments at other positions are smaller. Once blades are selected, the drag coefficients of both sides of blades are fixed. Therefore, the difference between work done in the rotor's left and right half circumferences decreases with the increase of rotor's rotational speed, i.e., the efficiency of drag-type rotor decreases with the increase of the rotor's rotational speed, which is opposite to the lift-type rotor.
Generally speaking, the wind rotor of a vertical axis wind turbine is composed of wing-shaped straight blades. The axial line of the blades is parallel to vertical axis. The wing shape of the blades can designed according to the current principles of aerodynamics. According to the principles of aerodynamics, the connecting line between the front and rear edges of blades is called the chord line and the rotating angle between blade's chord line and the tangent of the position on the circumference is set as rotating angle α. Normally, the blades are fixed on the cantilever support wing (cantilever support) or ring-type support rotating around vertical axis and cannot rotate relative to the cantilever support wing. That is to say that that blade rotating angle α is fixed and unchanged, as shown in FIG. 1. When strong enough wind blows over these straight blades that are wing-shaped and form a specific angle with the tangent of the rotating axis, a moment of rotation around the vertical axis is produced sufficient to rotate the entire wind rotor.
According to the principles of aerodynamics, the rotating angle between the chord line formed by connecting the centers of blades' front and rear edges and the wind direction is called the angle of attack δ, as shown in FIG. 4. Because the position of each blade in the circumference changes continuously, the blade's angle of attack changes accordingly during rotation. When a blade is at different positions during rotation, the size and direction of the driving moment produced by the blade continually changes due to changes in the blade's angle of attack, i.e., the produced moment of rotation is different. At certain positions, a larger driving moment is produced; at other positions, a smaller driving moment is produced. At certain positions, a large or small moment of resistance is produced. In conventional solutions, when a blade is at different positions during rotation, the blade rotating angle α is fixed and unchanged, while the size and direction of the driving moment produced by the blade constantly change. Therefore, it is unavoidable that conventionally blades are fixed on the wind rotor's cantilever support wing. This is an important factor affecting the utilization efficiency of the vertical axis wind turbine.